This invention relates to a process for the production of sodium dichromate and sodium dichromate solutions.
According to Winnacker-Kuchler, Chemische Technologie, Vol. 2, 4th Edition, 1982, pp. 657-664. sodium dichromate is produced by roasting of chrome ore with soda ash and/or sodium hydroxide and atmospheric oxygen under alkaline conditions.
A sodium monochromate solution containing from 300 to 500 g/l Na.sub.2 CO.sub.4 is obtained by leaching the furnace clinker leaving the roasting furnace with water or a chromate containing aqueous solution, adjusting the pH value of the mash from 7 to 9.5 and removing the insoluble constituents by filtration. The pH value is generally adjusted with sulfuric acid and/or with sodium dichromate solution. To produce sodium dichromate, the monochromate ions of the solution are converted into dichromate ions either by acidification with sulfuric acid, by acidification with carbon dioxide under pressure or by electrolytic acidification. Where sulfuric acid is used for acidification, sodium sulfate is formed and precipitates when the solution is concentrated to approximately 70% by weight Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O. The chromate-containing sodium sulfate obtained has to be worked up.
For acidification with carbon dioxide, the sodium monochromate solution is adjusted to a concentration of from 750 to 1,000 g/l Na.sub.2 CrO.sub.4 and is saturated with carbonic acid in autoclaves by introduction of carbon dioxide under a pressure of from 0.5 to 1.5 MPa (5 to 15 bar). 75 to 95% conversion of monochromate ions into dichromate ions is obtained with precipitation of sodium hydrogen carbonate. The remaining conversion to 100% can be obtained by introduction of carbon dioxide in another stage after concentration to 1,300 g/l Na.sub.2 CrO.sub.4 (already formed Na.sub.2 CrO.sub.7.2H.sub.2 O expressed in Na.sub.2 CrO.sub.4 equivalents). On an industrial scale, however, this final conversion step is very complicated. The final conversion step may also be carried out with sulfuric acid or by electrolysis.
As mentioned above, chromate-containing sodium sulfate is formed during the reaction with sulfuric acid and has to be worked up.
Electrolytic acidification is carried out by introduction of the sodium monochromate solution or rather the sodium monochromate solution converted to a level of from 75 to 95% into sodium dichromate into the anode compartments of electrolysis cells equipped with cation exchanger membranes. During the electrolysis process, sodium ions migrate through the membrane into the cathode compartments of the cells filled with water or with an aqueous solution. The sodium ions form an aqueous solution containing sodium hydroxide with the hydroxide ions formed at the cathode with simultaneous evolution of hydrogen. In the anode compartment, the chromate ions are converted into dichromate ions by the hydrogen ions formed at the anode with simultaneous evolution of oxygen. This process is also difficult to carry out on an industrial scale.
To produce sodium dichromate crystals, the sodium dichromate solutions obtainable in the processes mentioned above are concentrated by evaporation of water. The sodium dichromate which crystallizes is separated off and dried.
Now, the present invention adopts a different, simple approach to the production of sodium dichromate solutions and sodium dichromate crystals which does not have any of the disadvantages of the processes described above.